MX Lookup

Server assisted New DNS & Domains

Look up the MX records of a domain through Cloudflare or Google public DNS, sorted by priority, and resolve the first four mail servers to their A and AAAA addresses. The tool flags null MX, missing MX (implicit MX), mail servers that are aliases or IP addresses, and names that do not resolve.

Privacy: The name you look up is sent from the Mutqan server to the public DNS resolvers named in the results. Nothing is stored.

What is an MX record?

An MX (mail exchanger) record names a server that accepts e-mail for a domain, with a priority number: sending servers try the lowest number first and fall back to the others. The target must be a host name with its own A or AAAA records, a domain without MX falls back to its own address, and a null MX ("0 .") states that the domain accepts no mail at all.

How to use MX Lookup

  1. Enter the domain that receives the mail (for user@example.com, enter example.com — or paste the whole address).
  2. Choose the resolver if you want to compare Cloudflare with Google.
  3. Press Look up MX and read the table from the top: the first row is the server senders try first.
  4. Check the Note column and the findings for servers that do not resolve, aliases and missing IPv6.

MX Lookup features

  • MX records sorted by priority, with the remaining TTL of the set
  • IPv4 (A) and IPv6 (AAAA) addresses of the first four mail servers, resolved in the same run
  • Null MX (RFC 7505) recognised, and flagged when it is mixed with real mail servers
  • Implicit MX explained when a domain has no MX: the domain's own addresses that senders would use instead
  • Mail servers that are aliases (CNAME), IP addresses, names without addresses or private addresses called out
  • A whole e-mail address can be pasted — the domain after @ is used

Inputs and outputs

Inputs

  • Domain name — the mail domain, or a whole e-mail address whose domain part is used.
  • Resolver — Cloudflare 1.1.1.1 (default) or Google Public DNS.

Outputs

  • Summary — response code, number of mail servers, the preferred server, how many of the resolved servers have an IPv6 address (an AAAA record), and the lowest TTL.
  • Mail servers — priority, host name, IPv4 and IPv6 addresses and a note for each MX record.
  • Findings — null MX, implicit MX, aliases, unresolvable or private servers, shared priorities and single points of failure.

MX Lookup example

Mail servers of gmail.com

Input:

gmail.com

Output:

Response code: NOERROR · Mail servers: 5 · Preferred server: gmail-smtp-in.l.google.com · Servers with an IPv6 address: 4/4
5   gmail-smtp-in.l.google.com        142.251.127.27   2a00:1450:4001:c21::1b
10  alt1.gmail-smtp-in.l.google.com   142.250.147.26   2a00:1450:4025:c01::1b
20  alt2.gmail-smtp-in.l.google.com   172.253.152.26   2a00:1450:4010:c22::1a
30  alt3.gmail-smtp-in.l.google.com   192.179.20.26    2404:6800:4000:1027::1b
40  alt4.gmail-smtp-in.l.google.com   —                —   not resolved (first 4 only)
Findings: Senders try the lowest priority number first and move to higher numbers only when those servers cannot be reached.

How to read the MX Lookup results

Lower priority numbers win: the first row is where mail normally goes; the rest are fallbacks used only when the better servers cannot be reached.

"no A or AAAA record" or "name does not exist" next to a server means senders cannot use it; if it is the only server, mail to the domain fails. "unknown (lookup failed)" means the resolver gave no usable answer for that address query, so nothing is concluded — run the lookup again or switch resolver.

Null MX means the domain deliberately refuses mail. No MX at all is different: senders then try the domain's own address, which is usually a web server that does not accept mail.

IPv6 dashes are not an error — many mail servers are IPv4-only — but a server with only IPv6 cannot receive mail from IPv4-only senders.

When to use MX Lookup

  • Checking where a domain's mail goes before or after moving to Google Workspace, Microsoft 365 or another provider.
  • Debugging bounced mail: confirm that the MX hosts exist and resolve to public addresses.
  • Verifying that a website-only domain publishes a null MX so spoofed bounces are rejected quickly.
  • Collecting the mail servers' addresses for a firewall rule or an allow-list.

Troubleshooting

  • "Enter a domain name, not an IP address": MX records belong to domains; enter the part after @.
  • NXDOMAIN: the domain does not exist in DNS — check the spelling, or that the domain registration has not lapsed.
  • The old provider's servers still show after a migration: the resolver may hold the previous MX set until its TTL ends; compare resolvers or run the DNS Propagation Checker with type MX.
  • A server is marked as an alias (CNAME): replace the MX target with the final host name the provider documents.

Frequently asked questions about MX Lookup

What does the MX priority number mean?

Senders try the lowest number first and use higher numbers only when those servers cannot be reached. Servers that share a number get mail in random order, which spreads the load. The numbers are relative: 10 and 20 behave exactly like 1 and 2.

What happens when a domain has no MX record?

RFC 5321 tells senders to fall back to the domain's own A or AAAA address (the implicit MX), so mail may still arrive at the web server. The tool shows those addresses. If the domain should not receive mail, publish a null MX ("0 .") instead.

What is a null MX record?

A single MX record with priority 0 and the target "." (RFC 7505). It tells senders the domain accepts no mail, so they bounce at once instead of retrying for days — useful for domains that only host a website.

Can an MX record point to a CNAME or an IP address?

It should not. RFC 2181 requires the MX target to be a host name with its own A/AAAA records, and an IP address is not a valid target at all. Some senders tolerate an alias, others reject it, so the tool marks both cases.

Does this test whether the mail server accepts mail?

No. It reads DNS only: the MX records and the servers' addresses. It does not open an SMTP connection to port 25 — the Mutqan server never connects to port 25 — and it does not check SPF, DKIM or DMARC.

Technical notes

An MX record holds a preference and the host name of a mail exchanger (RFC 1035 §3.3.9). RFC 5321 §5.1 tells a sending server to sort the MX set by preference, randomise equal preferences, resolve each host to its addresses and fall back to the domain's own A/AAAA records only when no MX exists. RFC 2181 §10.3 forbids pointing an MX at an alias.

A null MX — preference 0, target "." — declares that the domain accepts no mail (RFC 7505). The tool makes one MX query plus, at most, an A and an AAAA query for each of the first four servers (or for the domain itself when there is no MX), all through the chosen public resolver.

Limitations and methodology

This is a DNS check only: it does not connect to the mail servers (port 25 is never contacted from the Mutqan server) and does not test SPF, DKIM, DMARC or TLS.

Addresses are resolved for the first four servers by priority; further servers are listed without addresses.

Addresses are what the public resolver returns from the Cloudflare edge; large providers may hand out different addresses in other regions.

Security and privacy

Only the domain is sent — never the part before @ — from the Mutqan server to the public resolver you chose, together with the mail server names it returns. Nothing is stored.

References